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Assessing the Quality of Carbon Nanotubes

We have assessed the quality of a commercial source of bulk multi-walled carbon nanotubes with respect to purity and degree of acid functionality. In addition, we have evaluated the applicability of several methods commonly used by suppliers to ascertain the purity of carbon nanotubes (SEM, TEM, and TGA). ICP-MS data indicate that residual metal catalyst is present, and is comprised primarily of nickel (1.2 weight %). It was also determined that most of the metal (> 99.5%) is encapsulated by carbon (likely amorphous carbon). Most of the catalyst was removed by acid oxidation, preceded by thermal treatment (400 – 500ºC in air) which liberated the metal from the encapsulating carbon. Based on mass loss during thermal oxidation, the amorphous carbon content was estimated to be approximately 45%. Titration analysis data of the as-received material indicate that the degree of acid functionality is 3.07 ± 0.27 mequiv/gram. This value decreases appreciably upon thermal treatment (2.56 ± 0.03 mequiv/gram at 500ºC, and 1.50 ± 0.05 mequiv/gram at 600ºC), which likely drives off the functional groups.